Organic EL display devices are known as thin, high picture quality, low power consumption display devices. In organic EL display devices, a plurality of pixel circuits including organic EL elements (also referred to as Organic Light Emitting Diode (OLED) or “display element”) that are self-luminous display elements driven by current and driving transistors are arranged in a matrix shape.
Configurations of pixel circuits in the related art included in such organic EL display devices will be described. FIG. 6 is a diagram illustrating a configuration of a pixel circuit 111 according to PTL 1, and FIG. 7 is a diagram illustrating a timing chart in order to drive the pixel circuit 111 illustrated in FIG. 6. As illustrated in FIG. 6, the pixel circuit 111 includes one organic EL element OLED, three transistors M1 to M3, a storage capacitor Cst, and a compensation capacitor Ccp. All these transistors M1 to M3 are N-channel type transistors. The transistor M1 is a driving transistor to control drive current to supply to the organic EL element OLED and is a transistor with a double gate structure including a bottom gate terminal Gb and a top gate terminal Gt. The transistor M2 is a writing transistor to charge the storage capacitor Cst with data voltage Vdata given to a data signal line Di in order to apply the voltage (data voltage) Vdata depending on data signals to the bottom gate terminal Gb of the driving transistor M1. The transistor M3 is a compensation transistor to charge the compensation capacitor Ccp for performing compensation for dispersion of threshold value voltage of the driving transistor M1 causing luminance variation. Dispersion of threshold value voltage of the driving transistor M1 is compensated by voltage charged in the compensation capacitor Ccp being applied to the top gate terminal Gt as back gate voltage Vbg.
An action of the pixel circuit 111 will now be described. As illustrated in FIG. 7, a potential of a first scanning signal line Saj connected to a control terminal of the writing transistor M2 changes from a low level to a high level in time t1. With this configuration, the writing transistor M2 turns into an on state, and a preset voltage Vpre is written in the storage capacitor Cst as data voltage Vdata through the data signal line Di. As a result, the preset voltage Vpre is given to the bottom gate terminal Gb of the driving transistor M1, and the driving transistor M1 turns into the on state.
At the same time, a potential of a second scanning signal line Sbj connected to the control terminal of the compensation transistor M3 also changes from a low level to a high level, and the compensation transistor M3 turns into the on state. At this time, power source voltage VDD of a high level power source line ELVDD connected to the driving transistor M1 maintains a high level. Therefore, current flows from the high level power source line ELVDD to a node N through the driving transistor M1 and the compensation transistor M3, and the compensation capacitor Ccp is charged with the power source voltage VDD of high level. The power source voltage VDD of high level charged in the compensation capacitor Ccp is applied to the top gate terminal Gt of the driving transistor M1 as the back gate voltage Vbg. At this time, voltage applied to an anode terminal and a cathode terminal of the organic EL element OLED is all high level because the power source voltage VDD of the high level power source line ELVDD and power source voltage VSS of a low level power source line ELVSS is all high level. Therefore, current does not flow in the organic EL element OLED.
At time t2, the writing transistor M2 turns into an off state by the first scanning signal line Saj changing from a high level to a low level. At this time, because the preset voltage Vpre is written in the storage capacitor Cst, the preset voltage Vpre is applied to the bottom gate terminal Gb of the driving transistor M1, and the driving transistor M1 turns into the on state. Because the power source voltage VDD of the high level power source line ELVDD changes from a high level to a low level, current flows from the compensation capacitor Ccp to the high level power source line ELVDD through the compensation transistor M3 and the driving transistor M1. With this configuration, the back gate voltage Vbg applied to the bottom gate terminal Gb begins to decrease, and threshold value voltage of the driving transistor M1 begins to rise.
Then, until time t3 when the preset voltage Vpre becomes not applied to the bottom gate terminal Gb of the driving transistor M1, current flows to the high level power source line ELVDD through the driving transistor M1, and the back gate voltage Vbg decreases accordingly. At time t3 when the back gate voltage Vbg becomes equal to threshold value voltage, current does not flow in the driving transistor M1, and the decrease in the back gate voltage Vbg also stops. Threshold value voltage of the driving transistor M1 is a predetermined value depending on the back gate voltage Vbg of this time, and its dispersion is compensated. For duration from time t2 to time t3, reverse voltage is applied to the anode terminal and the cathode terminal of the organic EL element OLED, and thus current does not flow in the organic EL element OLED, and the organic EL element OLED does not emit light.
At time t3, the potential of the second scanning signal lines Sbj changes from a high level to a low level, and the compensation transistor M3 turns into the off state. The power source voltage VDD of the high level power source line ELVDD changes from a low level to a high level, and the power source voltage VSS of the low level power source line ELVSS changes from a high level to a low level. Furthermore, the potential of the first scanning signal line Saj changes from a low level to a high level, and the writing transistor M2 turns into the on state. With these actions, drive voltage Vdrive depending on the data voltage Vdata is given from the data signal line Di. The drive voltage Vdrive is retained by the storage capacitor Cst and is applied to the bottom gate terminal Gb of the driving transistor M1. Therefore, current depending on the drive voltage Vdrive is supplied from the high level power source line ELVDD to the organic EL element OLED through the driving transistor M1, and the organic EL element OLED emits light with luminance depending on the drive voltage Vdrive.